The present invention relates to an electroluminescence element which is utilized as a still image or motion picture display means in a low-profile display device of a terminal of a computer system or the like.
FIG. 7 is a sectional view showing a conventional electroluminescence (to be abbreviated as an EL hereinafter) element of this type. As shown in FIG. 7, the conventional EL element is formed as follows. That is, a reflection preventive film 2 of SiO, MgO, or the like is formed on a transparent substrate 1 of a glass plate. Transparent electrode layers 3 of In.sub.2 O.sub.3, SnO.sub.2, or the like are aligned on the reflection preventive film 2. A first dielectric layer 4 of Y.sub.2 O.sub.3, Ta.sub.2 O.sub.5 or the like, an electroluminescent layer 5 of ZnS or the like in which 0.1 to 2 wt. % of Mn are doped as an activator, and a second dielectric layer 6 are sequentially stacked on the transparent electrode layer 3. Thereafter back electrode layers 7 of Al, Ta, Mo, or the like are aligned on the second dielectric layer 6. In this case, when viewed from the transparent electrode layer, a region where one transparent electrode layer and the corresponding back electrode layer crosses constitutes one pixel. When an AC voltage is applied between the electrodes, yellowish orange light having Mn as the activator is emitted from each pixel portion. Thus, display is made by controlling a voltage applied to the electrodes (e.g., refer to Japanese Patent Laid-Open No. 51-33579).
The reflection preventive film 2 in the conventional EL film adopts the principle that if the following thin film layer is interposed between two materials respectively having refractive indices of n.sub.1 and n.sub.2, reflectance with respect light of the wavelength .lambda. at the interface between the two materials becomes zero:
Refractive index: n=(n.sub.1 .multidot.n.sub.2).sup.1/2
Film Thickness: t=.lambda./4 (.lambda.: wavelength of light)
If the refractive index of the transparent substrate 1 is represented by n.sub.1, the refractive index of the transparent electrode layer 3 is represented by n.sub.2, and the central wavelength of light emitted from the electroluminescent layer 5 (to be referred to as EL light hereinafter) is represented by .lambda., the refractive index and the film thickness of the reflection preventive film 2 are selected to satisfy the above conditions. Then, the EL light from the electroluminescent layer 5 can be prevented from being reflected by the interface between the transparent substrate 1 and the transparent electrode layer 3. Thus, a decrease in effective luminance can be prevented.
The dielectric layer in the EL element is required to have high dielectric breakdown voltage and dielectric constant and small dielectric loss. In addition to these requirements, the first dielectric layer formed between the electroluminescent layer and the transparent substrate on which the transparent electrodes are formed is required to have a high adhesion force with the transparent substrate and transparent electrodes, and not to cause abnormality such as film cracking or peeling in a high-temperature heat treatment for activation after the electroluminescent layer is formed.
The conventional dielectric layer employs a single layer or multilayers of an oxide such as Y.sub.2 O.sub.3, Ta.sub.2 O.sub.5, Al.sub.2 O.sub.3, HfO.sub.2, PbTiO.sub.3, BaTa.sub.2 O.sub.6, or the like, or a material such as Si.sub.3 N.sub.4, silicon oxynitride, or the like. The layers of these materials are normally formed by the sputtering technique in order to prevent insulating breakdown due to microdefects.
However, the conventional EL element described above has the following problems:
(1) As described above, the refractive index n of the reflection preventive film 2 must satisfy the following relation if the refractive index of the transparent substrate 1 is represented by n.sub.1 and the refractive index of the transparent electrode layer 3 is represented by n.sub.2 : EQU n=(n.sub.1 .multidot.n.sub.2).sup.1/2
However, the transparent substrate 1 and the transparent electrode layer 3 can only employ very limited materials. The values of n.sub.1 and n.sub.2 are limited in advance by the materials which can be used. As a result, the value of n must be a limited, specific value derived from the values of n.sub.1 and n.sub.2. However, it is not easy to form a thin film having such a specific refractive index.
(2) In order to effectively apply a voltage applied between the transparent electrode layer 3 and the back electrode layer 7 to the electroluminescent layer 5, the specific dielectric constant of the layers interposed between the electrodes and the electroluminescent layer 5 must be increased as large as possible or their film thicknesses must be decreased, so that a voltage loss caused by a voltage drop across these layers is reduced as small as possible. However, the specific dielectric constant of a material normally employed for the reflection preventive film 2 is small (e.g., the specific dielectric constants of the above-mentioned SiO and MgO are respectively 4 to 6 and 9 to 10). In addition, in order to obtain the functions of the reflection preventive film, the reflection preventive film must have a thickness 1/4 the central wavelength .lambda. (in this case, about 1,500 .ANG.) of the EL light from the electroluminescent layer 5. For this reason, a voltage loss due to a voltage drop is considerably increased.
(3) The reflection preventive film 2 can provide a reflection preventive effect with respect to only light having the wavelength .lambda., i.e., the central wavelength of the EL light, and cannot provide the effect with respect to light of other wavelengths. Therefore, although the EL light from the electroluminescent layer 5 is efficiently output outside the layer, almost no reflection preventive effect can be obtained with respect to white light including various wavelengths externally incident on the EL element. Therefore, when the EL element is used in a bright location, the display is not easy to see due to reflection of external light.
(4) When the dielectric layer is formed by sputtering an oxide, the underlying transparent electrode may be darkened due to the influence of oxygen plasma, or an electrical resistance may be increased. Meanwhile, most compositions constituting the above-mentioned dielectric layer do not have sufficient adhesion force with the transparent substrate and electrodes. For this reason, peeling tends to occur by a heat treatment at a temperature of 400.degree. C. to 600.degree. C. performed for activating the electroluminescent layer. In order to solve this problem, the present inventors have already proposed a technique of preventing film peeling and degradation in the transparent electrode wherein an SiO.sub.2 film having good adhesion properties with the respective film layers is formed between the transparent substrate, the transparent electrodes and the dielectric layer in an argon gas atmosphere (Y. SHIMIZU, et al., CONFERENCE RECORD OF THE 1985 INTERNATIONAL DISPLAY RESEARCH CONFERENCE, P101, 1985). However, since the EL element with this structure has a large difference of refractive indices of the SiO.sub.2 film and the dielectric layer (e.g., if a BaTa.sub.2 O.sub.6 film is used as the dielectric layer, the refractive index of the dielectric layer is 2.4, while the refractive index of the SiO.sub.2 film is 1.4), a reflectance at their interface is increased, resulting in unclear display.